Atmospheric corrosion-resistant steel sheet for deep drawing



United States Patent O 3,303,060 ATMOSPHERIC CORROSION-RESISTANT STEEL SHEET FOR DEEP DRAWING Mineo Shimizu and Kameo Matsukura, Kitakyushu, Japan, assignors to Yawata Iron and Steel Co., Ltd., Tokyo, Japan, a corporation of Japan No Drawing. Filed May 28, 1963, Ser. No. 283,673 Claims priority, application Japan, June 5, 1962, 37/23 114 5 Claims. (oi. 148-121) This invention relates generally to improvements of extremely low carbon cold rolled steels or more particularly to deep drawing steel sheets having atmospheric corrosion-resistance and deep drawability.

An object of the present invention is to provide deep drawing cold rolled steel sheets which not only can be unprecedente-dly deep drawn but also have no tendency of orange peel and have good atmospheric corrosion-resistance.

Another object of the present invention is to provide an industrial method of producing deep drawing cold rolled steel sheets wherein atmospheric corrosion-resistance is attained by adding small amounts of phosphorus, copper and arsenic and the coarse grains caused by the decarburization annealing of sheets can be prevented by further adding a very small amount of any one or a combination of any two or more of titanium, aluminum,

zirconium and niobium. Another object of the present invention is to provide for automobile bodies deep drawing steel sheets which not only have good deep drawability and atmospheric but are not sufiicient for deep drawing and have no atmosphric corrosion-resistance. In order to improve deep drawability, it is necessary (1) to make the content of impurities in the steel as low as possible, (2) to regulate the crystal grains to be a proper size and (3) to arrange the preferred orientation of crystals in press working so that no press rejects may be caused by preventing the deformation in the direction of the thickness of the sheet as much as possible as compared with the deformation parallel to the surface of the sheet.

A so-called open coil annealing system wherein cold rolled steel coils are annealed in the state of loose coils 'has been recently developed. As a result, the content of carbon in a steel can easily be reduced to less than 0.01% and the deep drawability of steel sheets is remarkably improved by such removal and reduction of impurities, the crystal grains will grow unusually and thesurface of the steel sheet will become rough in press working. Further, the tensile strength of the steel sheet itself will be made so low because of decarburization annealing that the steel sheet will not be desirable as an iron and steel structure material.

According to the present invention, by taking the abovementioned points into consideration, there can be obtained steel sheets which not only can be unprecedentedly deep drawn but also do not have surface roughness after press work and have atmospheric corrosion-resistance.

The present invention shall be detailed in the following:

At ingot composed of less than 0.10% carbon, less than 0.08% silicon, O.20-O.60% manganese, 0.04 to 0.12% phosphorus, 0.200.60% copper, 0.030.5% arsenic and any one or a combination of any two or more of 0.005- 0.05% titanium, 0.0l0.05% aluminum, 0.0050.05% zirconium and 0.0050.05% niobium added, the rest being iron and unavoidably acompanying elements, is made by a steel manufacturing furnace such as an open-hearth furnace, electric furnace or converter.

In the above, the addition of copper or arsenic as well as phosphorus is necessary to give atmospheric corrosionresistance. In a steel containing 0.2-0.6% copper, if 0.03-0.l% arsenic is added, the atmospheric corrosionresistance can be further improved. 0n the other hand, when the copper is present only as an unavoidable impurity and only arsenic is added, 0.l0.5% arsenic must be added, because, in the case of adding arsenic only (not together with copper), with less than 0.1% arsenic, there will be a very small effect and, with more than 0.5% arsenic, the steel will become brittle. In the case of adding copper only, unless more than 0.2% copper is added, little effect will be obtained. On the other hand, if more than 0.6% copper is added, the steel will become hard.

Silicon is added as required for deoxidization. If less than 0.08% silicon is added, it will have no unfavorable influence on the deep drawability.

Manganese is necessary for deoxidization and for the elimination of the bad influence of sulphur present as an accompanying element in the steel. 0.200.60% manganese is proper. With more than that, the steel Will become hard. Any'one or a combination of any two or more of titanium, aluminum, zirconium and niobium is added to prevent crystal grains from becoming coarse in the final decarburization annealing of the rolled steel coil.

With the addition of titanium and the like in the abovementioned ranges, the object of the present invention can be attained. With more than the above-mentioned ranges, the surface condition will deteriorate and the steel will become so brittle as to be difficult to roll.

After the cast, the steel ingot is treated in the same manner as in the case of ordinary low carbon deep drawing steel. That is to say, a slab is made by slabbing the ingot and is hot-rolled to a fixed thickness. The hot rolled coil is then pickled and is cold-rolled at a reduction rate of 30 to 90% to a required final thickness. At the abovementioned reduction rate, the object of the present invention will be attained. However, in order to obtain a more preferred orientation and deep drawability, it is desirable to have a reduction rate in the range of 50 to 80%.

This cold rolled coil is then annealed while positively carrying out the decarburization and denitrification in a proper atmosphere, Le. a gaswhich will not oxide iron but will oxidize carbon contained in the iron as, for example, AX gas (cracked ammonia gas). In such case, when only the decarburization is to be carried out in the main, a gas mixture consisting of 3-20% hydrogen, the rest being mostly nitrogen, is used. When the decarburization is to be carried out together with the denitrification, a gas mixture consisting of more than hydrogen, the rest being mostly nitrogen, is used. When a fixed temperature (which is substantially a soaking temperature of 600- 760 C.) has been reached, steam is introduced to accelerate the reaction.

In the thus obtained steel sheet, the carbon content is usually less than 0.01%, high press forming workability Example 1 An ingot of a ladle composition of 0.06% C, 0.02% Si, 0.42% Mn, 0.092% P, 0.027% S, 0.040% Cu, 0.07% As, 0.008% Ti, 0.018% A1 and 0.0051% N was made in an open-hearth furnace. A cold rolled coil 0.8 mm. thick was made through required steps from the slabbing down to cold rolling of the ingot or such known steps as were described above. Said cold rolled coil was heated to 760 C. in an atmosphere of HNX gas 3-10 H and 0.05% of each of CO and CO the rest being N (dew-point of -40 C.). Steam was then introduced into the atmosphere to make a state of a dew-point of 40 C. The coil was heated in this state for 5 hours. The introduction of steam was then stopped. The atmosphere was again switched over to the dry HNX gas only. The coil was coo'led down to about 100 C. The chemical composition of the thus obtained product was 0.006% C, 0.018% Si, 0.42% Mn, 0.090% P, 0.025% S, 0.38% Cu, 0.07% As, 0.007% Ti, 0.017% A1 and 0.0050% N. The mechanical properties after the skin pass at a reduction rate of about 0.6% were a tensile strength of 32.5 kg./mm. a yield strength of 19.8 kg./mm. an elongation of 46.5%, an Erichsen value of 11.2 mm. a conical cup value of 35.76 mm., a grain size of No. 7.9-8.3 (A.S.T.M.) and a plastic strain ratio of 1.78 (rolling direction). Specifically the conical cup value was in the highest class, excellent deep drawability was shown, the grain size was proper and no surface roughness after press working was seen. Further, in the artificial ageing treatment at 100 C. for 1 hour, no deterioration of ductility was substantially seen.

On the other hand, in outdoor exposure tests for 290 days, whereas the amount of corrosion of an ordinary deep drawing steel sheet was 4.8 g./dm. that of the present invention was 2.6 g./dm. showing high atmospheric corrosion resistance. a

Example 2 An ingot of a ladle composition of 0.07% C, 0.01% Si, 0.38% Mn, 0.068% P, 0.020% S, 0.32% Cu, 0.08% As, 0.003% Ti, 0.015% A1, 0.018% Zr, 0.021% Nb and 0.0039% N, the rest being Fe and unavoidable impurities, was made with the oxygen top blowing converter. A cold rolled coil 0.8 mm. thick was made through required steps of slabbing, hot-rolling, pickling and cold rolling (at a reduction rate of 70%) of the ingot.

(of 5% H of a dew-point of -40 C. Then AX gas (ammonia decomposed gas) and steam were introduced 7 into the atmosphere to make a state of 22% H and a dew- Said cold rolled coil was heated to 740 C. in an atmosphere of HNX gas at a reduction rate of 0.6% were a tensile strength of 33.3 kg./mm. a yield strength of 20.6 kg./mm. an

elongation of'46.8% gauge length of 50 mm.), an Erich- Example 3 A steel sheet 0.8 mm. thick was obtained through the same treatment as in Example 2 from an ingot of a ladle composition of 0.06% C, 0.01% Si, 0.39% Mn, 0.071% P, 0.02% S, 0.30 Cu, 0.13% As, 0.031% Zr. and 0.0031% N, the rest being Fe and unavoidable impurities was made with the oxygen top blowing converter. The chemical composition of said steel sheet was 0.003% C, 0.011% Si, 0.38% Mn, 0.070% P, 0.019% S, 0.28% Cu, 0.13% As, 0.030% Zr and 0.0008% N, the rest being Fe and impurities. The mechanical properties after the skin pass at a reduction ratio of about 0.6% were a tensile strength of 32.1 kg./mm. a yield strength of 20.4 kg./mm. an elongation of 47.0%, an Erichsen'value'of 11.4 mm., a conical cup valve of 36.0 mm. and a grain size of No. 7.4- 8.0 (A.S.T.M.). No unusual surface roughness was seen after press work.

Example 4 A steel sheet was obtained through the same treatment as in Example 2 froman ingot of a ladle composition of 0.09% C, 0.01% Si, 0.38% Mn, 0.043% P, 0.015% S, 0.02% Nb and 0.28% As, the rest being Fe and unavoidable impurities was made with the oxygen top blowing converter. The chemical composition of said steel-sheet was 0.007% C, 0.01% Si, 0.39% Mn, 0.045% P, 0.014% S, 0.019% Nb and 0.28% As, the rest being Fe and unavoidable impurities. The mechanical properties after the skin pass at a reduction rate of a bout.0.6% were a tensile strength of 32.7 kg./rnm. a yield strength of 18.8 kg./mm. an elongation of 45.4% an Erichsen'value of 11.3 mm., a conical cup value of 36.2 mm. and a grain size of No. 7.8. In an outdoor exposure test for 290 days, the amount of corrosion was 2.6 g./dm.

Although we have described our invention with a certain degree of particularity, it is understood that the present disclosure have been made only by way of examples and that numerous changes in the details of construction and combination may be resorted 'to without departing from the spirit and the scope of the invention as hereinafter claimed. 7

What we claim is:

1. A steel sheet having atmospheric corrosion resistance and deep drawability consisting essentially of less than 0.01% C, less than 0.08% Si, 0.200.6% Mn, 0.04- 0.12% P, 0.20.6% Cu, 0.030.5% As and at least one member selected from the group consisting of 0.005- 0.05% Ti, 0.01-0.05% Al, 0.0050.05% Zr and 0.005:- 0.05% Nb, the remainder being Fe and unavoidable impurities.

2. A process for producing steel sheet having atmospheric corrosion resistance and deep drawability comprising steps of making an ingot consisting essentially of less than 0.10% C, less than 0.08% Si, 0.20 060%. Mn, 0.040.12% P, 0.200.60% Cu, 0.03 0.5% As and at least one member selected from the group consisting of 0.005-0.05% Ti, 0.010.05% Al, 0.0050.05% Zr and 0.00-5-0.05% Nb hot and cold rolling said ingot to produce a cold rolled coil, annealing said cold rolled coil in a decarburizing atmosphere gas in an open coil annealing apparatus and recrystallizing, softening and annealing said coil while decarburizing and denitriding same so as to produce a non-ageing steel containing less than 0.01% C.

3. A process for producing steel sheets having atmospheric corrosion resistance and deep drawability according to claim wherein the cold rolling is carried out at a reduction rate of 50 to V ,7 v

4. A process for producing steel sheets having atmospheric corrosion resistance anddeep drawability accord- 5 coil annealing is a gas mixture consisting of more than 70% hydrogen, the remainder being substantially nitrogen, and, when a fixed temperature has been reached, steam is introduced into the atmosphere to acelerate the reaction.

References Cited by the Examiner UNITED STATES PATENTS 2,056,591 10/1936 Schulz et a1. 75-124 6 Holzwarth et a1. 75123 Low 148-16 Beall 14816 Olt et a1. 14816 Kopchack 75123 X Yoshida 14812 X HYLAND'BIZOT, Primary Examiner.

DAVID L. RECK, Examiner.

8/1952 Farling et a1. 14812 10 H. F. SAITO, Assistant Examiner. 

2. A PROCESS FOR PRODUCING STEEL SHEET HAVING ATMOSPHERIC CORROSION RESISTANCE AND DEEP DRAWABILITY COMPRISING STEPS OF MAKING AN INGOT CONSISTING ESSENTIALLY OF LESS THAN 0.10% C, LESS THAN 0.08% SI, 0.20-0.60% MN, 0.04-0.12% P, 0.20-0.60% CU, 0.03-0.5% AS AND AT LEAST ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF 0.0005-0.05% TI, 0.01-0.05% AL, 0.005-0.05% ZR AND 0.005-0.05% NB HOT AND COLD ROLLING SAID INGOT TO PRODUCE A COLD ROLLED COIL, ANNEALING SAID COLD ROLLED COIL IN A DECARBURIZING ATMOSPHERE GAS IN AN OPEN COIL ANNEALING APPARATUS AND RECRYSTALLIZING, SOFTENING AND ANNEALING SAID COIL WHILE DECARBURIZING AND DENITRIDING SAME AS TO PRODUCE A NON-AGEING STEEL CONTAINING LESS THAN 0.01% C. 